Split Lock Washer (Helical Spring)
Direct Answer
The Split Lock Washer (Helical Spring Washer, standardized under DIN 127 and ASME B18.21.1) is a split ring formed into a helical spring shape with trapezoidal or rectangular cross-sections. When compressed flat under a bolt head, it exerts continuous axial spring pressure to compensate for minor settling and prevent loosening under low-vibration loads.
Source: DIN 127 (Form A / Form B) / ASME B18.21.1 (Helical Spring Washers)
Specs
| Standard Specifications | DIN 127 (Form B flat end / Form A bent tang) / ASME B18.21.1 / DIN 7980 (Socket caps) |
| Spring Form Geometry | Single-coil helical ring with cut split gap and opposed offset ends |
| Spring Mechanism | Elastic axial spring reaction force when compressed flat |
| NASA / Engineering Limitation | Ineffective in high-vibration severe dynamic joints (NASA-RP-1228 flags split washers as useless under severe vibration) |
| Optimal Joint Types | Low-vibration electrical terminal studs, low-tension assemblies, wood expansion take-up |
Operating Mechanism and Split Edge Bite
When tightened, the helical ring is compressed flat into the joint. The sharp, split cut ends of the ring dig slightly into the underside of the bolt head and mating workpiece surface, resisting counter-clockwise rotation.
The Modern High-Vibration Controversy
Extensive dynamic testing (Junker Vibration Test) and NASA Fastener Design Standard (NASA-RP-1228) demonstrate that once a high-tensile bolt is fully torqued flat, a split lock washer behaves merely as a weak spring with a fraction of the bolt's clamping load. If dynamic vibration overcomes bolt preload, the split ends can actually act as a friction-reducing bearing, allowing the bolt to unwind faster than without a washer. For critical joints, Nyloc nuts, threadlocking fluid (Loctite), or Nord-Lock wedge-locking washers are mandated.